Occupants in a room may begin to feel effects when the oxygen level drops to what percentage?

Occupants in a room may begin to feel effects when the oxygen level drops to around 15% oxygen because this level is significantly lower than the normal atmospheric oxygen concentration of approximately 21%. At 15% oxygen, individuals may experience symptoms such as fatigue, difficulty in concentrating, and shortness of breath. Prolonged exposure to below-normal oxygen levels can lead to more severe health issues, including loss of consciousness and potential long-term damage.

While the other percentages listed can also be hazardous, the onset of noticeable symptoms typically occurs around the 15% mark, making it the correct choice in this scenario. This illustrates the importance of maintaining adequate oxygen levels in confined spaces to ensure occupant safety and well-being.